RALEIGH — A team of academic researchers equipped with a drone estimates that up to 35 million gallons of coal ash and contaminated wastewater spilled into the Dan River earlier this month.

Researchers at Wake Forest University’s Center for Energy, Environment and Sustainability released results Tuesday from a study using photos collected by the drone that flew over the Duke Energy coal ash dump that ruptured Feb. 2 in Eden. The scientists used images captured by the drone to create a digital three-dimensional model of the pit, allowing them to calculate the volume of toxic ash that flowed out when a pipe collapsed.

The reported amount equals about 53 Olympic-sized swimming pools, making it the third-largest coal ash spill in the nation’s history.

“We think we have a precise estimate of the ash and water slurry that flowed out of the pond based on our methodology,” said Miles Silman, a biology professor at Wake involved with the project. “The uncertainty comes from the amount of water that continued to drain from the ash, which is a question for hydrologists. Our work provides an independent estimate of the spill.”

The researchers’ calculations put the size of the spill within the broad range that Duke’s engineers initially estimated, but larger than a revised estimate later cited by company executives.

The Wake Forest survey team has shared its results with federal and state regulators, as well as Duke Energy.

Company spokesman Tom Williams declined to comment on the new study.

An accurate calculation of the size of the spill could be key to determining the amount of whatever fines the company may be assessed for the spill, which authorities say coated the river bottom with gray sludge as far as 70 miles downstream. Coal ash contains numerous chemicals that are toxic to humans and wildlife, including lead, arsenic and mercury.

Federal prosecutors have opened a criminal investigation in the wake of the spill, issuing more than 20 subpoenas to Duke and state regulators.

“We want to be completely transparent in this,” Silman said. “Our goal is to help stakeholders deal with the spill and its consequences, and show the technology as a cheap and cost-effective way to monitor the environment.”